1. Field of the Invention
This invention relates to a plunger lock of the type having an elongate shank or housing with an enlarged head at one end and at least one lock element movable to project from the housing to engage a grooved or recessed member and lock the same to the housing and to retract into the housing to release the grooved or recessed member, and to a key for operating the lock.
2. Description of the Prior Art
It is known from U.S. Pat. No. 3,033,016 to provide a lock of the class described to hold in locked relationship two members having apertures which may be moved into alignment. The housing of the lock may be inserted through the apertures and a cap having an internal groove may be applied endwise over the housing so that the lock element can enter the grooves and be retained there until released by action of a suitable key, the head and cap being larger than the apertures.
This patent also discloses a key having two adjacent, projecting fingers so formed that when one is moved axially relative to the other, the distal ends of the fingers are spread apart to engage the surface of the bore of a lock operating member to shift the same against a spring force to unlocking position.
U.S. Pat. No. 3,002,368 discloses a similar lock wherein a plunger 52 axially movable in the housing to project or release lock elements 36 relative to the housing, may tilt relative to plug 58 to engage the same and prevent unauthorized retraction and release of elements 36, as shown in FIG. 4.
U.S. Pat. Nos. 3,554,218 and 3,560,130 and application Ser. No. 661,118 filed Oct. 15, 1984 are of interest and disclose various ways in which locks of the class described can be used. Thus, it will be noted that a cup shaped head can be locked to the end of the plunger type lock to secure valve parts against relative movement as in FIG. 2 of U.S. Pat. No. 3,554,218 or the ball bearings may engage recesses in one of the parts so that a head is not necessary, as in FIGS. 1 and 4 of U.S. Pat. No. 3,560,130. In another application, as in FIG. 7 of the last-integrated with one of the valve parts.
Another lock disclosed in U.S. Pat. No. 4,519,225 uses a center pin 68 with an enlarged head 70 at its front end and a rearwardly extending shaft 72 extending through a bore 58 in the plunger 44. This pin head 70 fits slidingly in barrel bore 34. A friction producing compound is placed in the closed front end of barrel 22 to hold the head against easy movement in the barrel when an authorized key is used to retract the plunger 44, the compound exerting sufficient frictional force on head 70 to prevent axial movement of the pin 68. On the other hand, when an unauthorized device is forced into bore 56 it becomes impaled on pin 68 which moves with plunger 44 causing head 70 to position itself beneath lock members 16 holding them projected and the device in locked condition.
A barrel lock which uses a similar headed pin and a friction producing compound is disclosed in U.S. Pat. No. 4,513,592 and other disclosures of interest will be found in U.S. Pat. Nos. 3,835,674, 4,015,456, 4,058,992, 4,015,456 and 4,543,807.
A commercially available lock known as the Brooks Barrel Lock manufactured by G. J. Brooks company is shown in FIG. 1 of the drawing and will be explained later.
Thus, it will be appreciated that a number of attempts have been made to improve the security of locks of the class described. Nevertheless, the locks disclosed in the foregoing patents can be defeated by jamming a nail or the like into the area of the bore in the plunger to wedge the nail in the bore sufficiently to withdraw the plunger and unlock the device. U.S. Pat. No. 4,543,807, discusses methods sometimes used to defeat a lock of the class described. One such method involves the use of a quick setting adhesive such as an epoxy or cyano-acrylate and, while the patentee indicates that with his device this method will not succeed because the adhesive will spread out adhering the pin 26 and plunger 22 together, a condition which will prevent the device from unlocking, even in this device this adhesive technique may succeed if an excessive amount of adhesive is not used.
It will also be appreciated that most of the proposed solutions to unauthorized manipulation of locks of the class described involve structure having a relatively large number of parts which leads to high cost of manufacture and assembly.